The present invention relates to a drive circuit for an image point of an image screen, which has in particular an organic light-emitting diode, with a capacitor and a current feedback, wherein a first thin film transistor is provided as the current driving transistor for the diode, and a second thin film transistor is connected with a current-conducting electrode with a gate of the first transistor and with a second current-conducting electrode with a data conductor and with its gate electrode with a scanning signal conductor.
In the driving of image screens with light-emitting diodes (LED), in particular organic, light-emitting diodes (OLED) via thin film transistors, spacial fluctuations of the LED driver currents occur because of manufacturing-dependent fluctuations of the parameters of the thin film transistors, in particular the threshold voltage and the charge carrier movement. Thereby disturbing spacial inhomogenuities of the image screen light density are caused.
In order to solve this problem, various compensation features for the driver current fluctuations of the LED are proposed. A. Yumoto, et al discloses in “Pixel-Driving Methods of Large-Sized Poly-Si AM-OLED displays”, Asia Display/IDW'01, pages 1395-1398, 2001 the driving circuits typically with at least four thin film transistors for compensation of the fluctuations of the driving currents. These circuits provide however only a partial compensation and therefore, with a great number of transistors, produced a relatively low manufacturing yield.
U.S. patent application U.S. 2002/0101172 A1 discloses a driving circuit which additionally has further thin film transistors for supplying the LED current back to an external current-voltage conversion circuit and therefore allowing a feedback of the actual flowing current.
The known voltage-control solutions allow however the compensation of the threshold voltage fluctuations, but not also the compensation of fluctuations of the charge carrier movement. The current-controlled solutions are very high-ohmic and require relatively long response times. With the use of pure current mirror circuits, two thin filmed transistors must have approximately identical properties, that is difficult to implement for thin filmed transistors.
A further disadvantage of the known, above described current feedback circuit is that parts of the drive circuit must be realized at both sides of the LED element, that requires a technically extremely difficult-to-produce through contacting with the LED semiconductor material; in particular with organic semiconductor materials. Moreover, the known circuit is expensive since four additional thin filmed transistors are required, including two thin film transistors which act as switches and two thin film transistors for an invertor.